2. Detailed Data Description

Introduction

The Nimbus-7 SMMR Pathfinder Brightness Temperatures data set contains global
brightness temperatures in swath format (level 1b) from October 25, 1978 to
August 20, 1987. The instrument obtained near-global coverage at five frequencies
(6.6, 10.7, 18, 21, and 37 GHz) in both horizontal and vertical polarizations, at
a constant incidence angle of 50.3 degrees, every six days. Data are stored as
daily orbit files in compressed Hierarchical Data Format (HDF) and are available on FTP.

The data set was produced at the Jet Propulsion Laboratory (JPL) from a
level 1A antenna temperature ('TAT') data set provided by A. Chang and D. Han of the Goddard
Space Flight Center (GSFC). The TAT data consisted of uncalibrated raw sensor output.

The SMMR Level 1B data set contains global brightness temperatures
spanning October 25, 1978 to August 20, 1987. The
SMMR was launched by NASA on the Nimbus-7 satellite, and measured the
Earth's microwave radiation at five frequencies (6.6, 10.7, 18, 21, and
37 GHz) in both horizontal and vertical polarizations at a constant
incidence angle of 50.3 degrees. Near-global coverage was obtained
every six days. Details of the SMMR instrument, the data processing
algorithms, and early geophysical results are provided by Gloersen and Barath (1977), Njoku et
al. (1980), Swanson and Riley (1980), Njoku (1980), Gloersen et al. (1984), Gloersen (1987) and
Fu et al. (1988).

This data set is the outcome of a project to reprocess the original level
1B data. The primary objectives were to remove (or reduce) known
calibration anomalies in the data (Milman and Wilheit 1985, Liu and Mock 1986, Frances 1987,
and McMillan and Han 1990) and to provide a
globally-consistent data set, available on compact media, for global
change research (ocean, land, atmosphere, and cryosphere). The
processed data are in swath format with the original sampling and
resolution of the data maintained. Processing details
provided by Njoku et al. (1995). The data are available in the Hierarchical Data Format (HDF).

Each data file contains a complete orbit, defined as beginning and
ending at successive descending node equator crossings (approximately
midnight local time). The orbital period is approximately 104.16
minutes, resulting in approximately 13.8 orbits per day. The file size
for an uncompressed SMMR orbit file is 4.3 megabytes. Due to spacecraft
power constraints, the SMMR was limited to acquiring data on alternate
days for most of the mission. Additional time gaps occur in the data,
primarily due to gaps in acquisition, transmission, or processing of the
raw data. The size of the entire (approximately nine-year) data set is 59
gigabytes compressed, and 101 gigabytes uncompressed.

Objective/Purpose

Production of this data set was motivated by the desire to improve upon
earlier versions of archived Nimbus-7 SMMR brightness temperatures by
providing: (1) data corrected to the extent feasible for observed
calibration anomalies in all radiometer channels; (2) data processed
globally in swath format at the original sensor sampling and resolution
(i.e. without gridding to Earth-fixed coordinates); (3) data processed
for the full mission duration using a uniform set of processing
algorithms and procedures; (4) data archived, documented, and
accessible, on compact media and in standard format, and available at an
EOSDIS DAAC, for use in global change research.

Summary of File Variables

Table 1 provides a brief description of the information and variables contained in the HDF data files. For complete details, see the SMMR Level 1B Pathfinder Data Set document.

Contains spacecraft information such as yaw, pitch, and roll and sun ascension and declination information as well as date and time. See section 3.2 in SMMR Level 1B Pathfinder Data Set for complete details.

Contains the latitude and longitude of the data as well as antenna scan angles, reflected sun-footprint angles, and footprint incidence angles. See section 3.4 in SMMR Level 1B Pathfinder Data Set for complete details.

Contains an indicator of the surface type based on the radiometric data latitudes and longitudes in
conjunction with the Fleet Numeric Meteorological and Oceanographic Center SSM/I surface type database. See section 3.7 in SMMR Level 1B Pathfinder Data Set for complete details.

In addition, daily, global browse images of selected brightness
temperature channels are available as follows:

6.6 GHz Horizontal Ascending (Day)

6.6 GHz Horizontal Descending (Night)

37 GHz Horizontal Ascending (Day)

37 GHz Horizontal Descending (Night)

Discussion

This data set is unique in that it contains brightness temperatures
processed in swath format at the original instrument sampling
resolution. It includes corrections for instrument calibration
anomalies found to be present in data processed originally by the Nimbus
Satellite Project. Other reprocessed data sets have also been published,
using subsets of the SMMR channels in Earth-gridded format and over
specific geographic regions (Wentz and Frances 1992, Gloersen et al. 1992). Comparisons of the
present data with these other data sets has not been performed as of the publication date of this
Guide (September, 1995).

Spatial Characteristics

Spatial Coverage and Resolution

The data set extends from latitudes of 84
degrees South to 84 degrees North and longitudes of 180 degrees West to
180 degrees East. The 50 degree scan provided a
780 km swath of the Earth's surface.

Figure 1. Spatial Coverage Map

Projection

Temporal Coverage

The data set covers 25 October 1978 to
20 August 1987. The SMMR operated continuously during the three week
checkout period from October 25, 1978 to November 16, 1978. After that,
the instrument was switched on and off on alternate days.
From April 3 to June 6, 1986, a special operation
was under way during which the SMMR was switched off more frequently.
The off periods averaged length of 75 minutes and the on periods
averaged 30 minutes.

Temporal Coverage Graph

The following graphic shows the percentage of missing data
for the SMMR instrument. The value for 1986 does not include the period
of special operation which was from April 3 to June 6, 1986.

Figure 2. Temporal Coverage Graph

Temporal Resolution

Switching of the instrument on and off on
alternate days was the normal mode of operation for the SMMR for most of
its mission due to power sharing constraints among instruments on the
spacecraft. The SMMR was routinely turned on at close to midnight GMT
(corresponding to a descending node equator crossing near 0 longitude),
and turned off at approximately the same time the following day.
The satellite completes nominally 13.8 orbits in a day.
Each level 1B orbit file covers a time period of approximately
104.16 minutes.

Data Characteristics

Parameter/Variable

Brightness Temperatures

Variable Description/Definition

Brightness temperature is the effective temperature of a
blackbody radiating the same amount of energy per unit area at the same
wavelengths as the observed body.

Unit of Measurement

All temperatures are stored in Kelvins * 0.1.

Data Source

The SMMR Pathfinder brightness temperature data set was
generated from SMMR measurements.

Data Range

The range for the brightness temperatures is
approximately 65 K to 325 K.

Data Granularity

A general description of data granularity as it applies to the IMS appears in the
EOSDIS Glossary.

The data are stored in daily files by orbit.
A complete orbit is defined as beginning and
ending at successive descending node equator crossings (approximately
midnight local time). The orbital period is approximately 104.16
minutes, resulting in approximately 13.8 orbits per day.
The file size for an
uncompressed orbit file is approximately 4.3 megabytes. Each daily file is
composed of all the orbits for that particular day.

Data Format

The data are stored in Hierarchical Data Format (HDF).

3. Data Access and Tools

Data Access

Software Tools

4. Data Acquisition and Processing

The input data for the Pathfinder SMMR brightness temperature
data processing are the level 1A antenna temperature tapes (TAT)
provided by A. Chang and D. Han of GSFC. The TAT data were produced and
archived originally by the Nimbus-7 Satellite Project at GSFC.

Sensor or Instrument Description

The SMMR instrument employed a 79-cm diameter offset-fed parabolic
reflector to direct Earth-emitted radiation into a 10-channel
multi-frequency feed horn. At the ports of the feed horn the radiation
was split into the designated frequency and polarization channels
(frequencies of 6.6, 10.7, 18, 21, and 37 GHz, each with horizontal and
vertical polarizations) and transmitted via wave guide or coaxial line to
the respective radiometers for detection. Scanning was accomplished by
sinusoidal oscillation of the parabolic reflector (offset at 42 degrees
from nadir) about a vertical axis with a period of 4.096 seconds. The
antenna orientation was forward-looking, with a symmetric angular scan
between azimuth angles of +25 and -25 degrees to each side of the
suborbital track. This resulted in a conical scan, with a constant
incidence angle at the Earth's surface of 50.3 degrees. The scan
pattern mapped out a swath width of 780 km at the Earth's surface.

The spatial resolutions at the various frequencies ranged from
approximately 27 km at 37 GHz to 148 km at 6.6 GHz. The radiometer
sensitivities ranged from 0.7 K at 6.6 GHz to 1.4 K at 37 GHz, for
radiometer integration times ranging from 128 to 32 ms, respectively.
Calibration data were acquired at the scan extremes by alternately
switching the radiometers to sky horns viewing cold space and internal
calibration loads at the instrument ambient temperature. A summary of
the nominal SMMR operating characteristics is shown in Table 1. Further
details on the instrument design and operation are given in Refs [2] and
[3].

Source/Platform

The Nimbus-7 satellite was launched by NASA on October 24, 1978 into a
near-polar sun-synchronous orbit of altitude 955 km and inclination of 99
degrees. The orbital period was 104.16 minutes, with a nodal
progression of 26.1 degrees westward, resulting in approximately 13.8
orbits per day. The nodal crossings were at local noon (ascending) and
local midnight (descending). There were eight scientific instruments on
the spacecraft including the SMMR. Due to spacecraft power limitations
only one of the instruments operated on a full 100% duty cycle. The
SMMR was constrained to a 50% duty cycle, and was turned on and off on
alternate days.

The attitude control system was designed to maintain spacecraft
stability to within 0.7 degrees in pitch, and 1 degree in roll and yaw.
Nominally, a sun sensor was used to sense the spacecraft attitude during
daytime portions of the orbit, and horizon sensors were used during
nighttime portions. Spacecraft pitch, roll, and yaw angles, and
ephemeris data, were used to compute footprint latitude and longitude
locations and incidence angles. These data were placed on the TAT data
record. Further information on the Nimbus-7 platform and instruments
can be found in Ref. [16].

Source/Platform Mission Objectives

The SMMR was an experimental Earth-imaging passive microwave sensor
launched as a follow-on to earlier experimental radiometers (ESMR, NEMS,
and SCAMS) launched by NASA on previous satellites in the Nimbus series
(Nimbus-5 and -6). The SMMR was designed primarily for remote
observations of oceanic, cryospheric, and tropospheric moisture-related
phenomena. Key geophysical variables observable by the SMMR included
sea surface temperature, sea surface wind speed, columnar water vapor
over the ocean, cloud liquid water over the ocean, rain rate, sea-ice
concentration and type, snow extent and depth, and potentially other
land parameters such as soil moisture, surface temperature, and
vegetation extent.

Key Variables

Principles of Operation

Six conventional Dicke-type radiometers are used. Those operating at the four longest
wavelengths measure alternate polarizations during successive scans of the antenna; the others,
at the shortest wavelength, operate continuously for each polarization. A two point reference
signal system is used, consisting of an ambient RF termination and a horn antenna viewing
deep space. A switching network of latching ferrite circulators selects the appropriate
polarization or calibration input for each radiometer.
For more details on the principles of operation of the SMMR instrument,
see the
Scanning Multi-channel Microwave Radiometer
guide document.

Manufacturer of Sensor/Instrument

Final design and fabrication was accomplished at the Jet Propulsion Laboratory by a
team under the direction of J. Johnston.

Derivation Techniques and Algorithms

For information on the derivation techniques and algorithms,
see Gloersen (1987) and Fu et al. (1988).

Data Processing Sequence

A description of the theory and implementation of the processing is
given in Njoku et al. (1995). Care was taken to ensure that instrument-related
effects (and not geophysical signatures) were removed in the processing.
The approach synthesized results and analyses from previous SMMR
calibration studies (Swanson and Riley 1980, Njoku 1980, Gloersen 1987, Fu et al. 1988, Frances
1987, McMillan and Han 1990, Gloersen et al. 1992).

The following were the main steps in the procedure:

(1) Calibration of the radiometric data to convert from raw counts to
antenna temperatures.

The equation converted radiometer counts to antenna temperature,
making use of the cold sky and warm load calibration counts. Terms in
the equation included off-line-derived coefficients to account for
losses in the component waveguides, horns, and switches, and in-orbit
engineering temperatures measured by thermistors mounted on these
components (Fu et al. 1988).

Anomalous cold-sky calibration counts were recorded by the SMMR during a
period each orbit when solar radiation entered the sky horns. This
occurred over a time interval of about one-sixth of each orbit, and
resulted from the spacecraft's orientation relative to the sun as a
function of orbit position. The anomalous data were corrected by
replacing them with cold counts interpolated linearly between the
weighted-average values immediately before and after the anomaly
periods. (To reduce effects of noise and other occasional errors in
calibration counts and component temperatures, these data were smoothed
by computing them as weighted running-averages, and by using thresholds
to exclude large error values.)

(a) Adjustments for calibration bias errors were performed in a manner
similar to that described by Gloersen (1987). A renormalization was done to
correct for portions of the antenna sidelobes viewing cold space. A
linear correction was made to the calibration equation in order to
adjust the observed ocean brightness temperatures to those computed from
a radiative transfer model (Hofer and Njoku 1981) for similar ocean-atmosphere
conditions. This was done by using modeled brightness temperatures,
with averaged ocean and atmosphere climatological conditions as inputs,
as a "cold temperature" reference point, and the internal warm load as a
"warm temperature" reference point. The assumed climatological
conditions were typical of average conditions for December in the
latitude range 40 to 50 degrees South. Averaged SMMR data from December
1978 nighttime passes over ocean in the latitude range 40 to 50 degrees
South were used to obtain the coefficients for the linear adjustment.
Two passes of this procedure were performed to minimize the effects of
clouds.

(b) Corrections for long-term calibration drifts were implemented in a
manner similar to the procedures described in Frances 1987, McMillan and Han 1990, and
Gloersen et al. (1992).

Six-day global ocean averages of SMMR brightness temperatures
(away from land or ice) were generated for the duration of the mission.
The annual cycle was removed, and the residual trends in each channel
were fit by least squares to polynomial functions of time. These
polynomials were applied as corrections for long-term drift at the "cold temperature" reference of the calibration equation. In applying this
approach, it was assumed that the global ocean average brightness
temperatures (with the annual cycle removed) should be approximately
constant with time, and that any residual, secular long-term trends,
uncorrelated by radiometer channel, could be ascribed to calibration
drifts, such as due to gradual degradations of instrument components. A
correction for long-term drift was not applied at the "warm temperature"
reference of the calibration equation since averaged land brightness
temperatures, which might be appropriate as a warm calibration
reference, do not exhibit the temporal stability of averaged ocean
temperatures. However, degradations of instrument components, leading
to increased resistive losses, normally have much larger relative
effects at cold brightness temperatures than at warm brightness
temperatures. Hence, most probably this is not a serious omission.

On January 4, 1984, the recorded SMMR incidence angles and some of the
brightness temperatures (predominantly the vertical polarizations)
exhibited small but significant discontinuous jumps in their mean
values. (The mean recorded incidence angle changed from 50.2 degrees to
49.8 degrees.) The nature and magnitudes of these jumps appeared to be
consistent with the possibility that a discontinuous change in the mean
spacecraft attitude had occurred. However, these jumps were not
discovered in the SMMR data until after the mission when long-term trend
analyses were performed, and the cause could not be determined
retrospectively, and conclusively from the spacecraft and instrument
data records. For this reason, the long-term trend polynomial fits
described above were computed and applied in two segments - before and
after January 4, 1984. Between these two segments, the fitted trends
exhibited the discontinuities shown in Table 2. It is recommended that
the brightness temperature "delta" values shown should be added as a
calibration offset to all ocean brightness temperatures subsequent to
the January 4, 1984 date. These offset adjustments were not applied
automatically during reprocessing of the data since there was
uncertainty as to whether their cause was in fact instrument-related,
software-related, or geophysical in nature.

(c) Calibration drifts with respect to the so-called "ecliptic" angle
(the angular distance of the spacecraft in its orbit from the point of
closest approach to the sun) were caused by variations in solar heating
of the SMMR instrument at different positions in the orbit. The solar
heating induced dynamic temperature gradients in the horns, waveguides,
and other instrument components, that could not be fully compensated by
the basic calibration equation. The calibration drift characteristics
were repetitive each orbit since the Nimbus-7 orbit was sun-synchronous.
The drifts were estimated and corrected using a procedure similar to
that described by Frances (1987) (and also used by Gloersen et al. 1992). In this
procedure, differences between co-located brightness temperatures from
ascending and descending orbital crossings over the ocean were
accumulated for each year, averaged by latitude band (related to
ecliptic angle) and six-day period. It was assumed that, averaged over
the year, these differences should be zero, and that departures from
zero represented calibration drift errors. A weighted least squares
procedure was used to estimate these errors as a function of ecliptic
angle for each year of the mission. The functional dependence varied
slightly from year to year, hence a linear interpolation between years
was computed. The final corrections were used as adjustments to the
"cold temperature" reference in the calibration equation.

(3) Interpolation of the radiometric data for all channels to the
locations of the 37 GHz vertical radiometric data.

As described by Njoku (1980), the sampling sequences of the SMMR radiometers
were non-coincident. Also, the vertical and horizontal polarizations
(except at 37 GHz) were sampled on alternate halves of the oscillating
scan cycle. In order to correct for polarization mixing of the
brightness temperatures co-located vertical and horizontal data at each
frequency were required. This was accomplished by interpolating the
measured brightness temperatures along- and across-scan so that vertical
and horizontal channels were co-registered at each frequency. In actual
implementation all channels were in fact interpolated to be co-
registered to the locations of the 37 GHz vertical samples. This
additional density of interpolation provided uniform, co-located data
arrays at each channel, and simplified the further processing steps
considerably. However, it should be remembered that the interpolation
does not increase the inherent resolution of the data channels (as given
in Table 1). Conversely, the interpolations are over distances that are
relatively small with respect to footprint size, and hence should not
adversely affect the accuracy or fidelity of the data.

(4) Correction of the antenna temperatures for polarization mixing.

Mixing of the received vertical and horizontal brightness temperatures
occurred due to antenna effects (cross-polarization and scan-induced
polarization coupling) and also due to polarization switch leakage.
Since these effects, in particular the switch leakages, could not be
adequately characterized prior to launch, correction coefficients had to
be estimated and applied post-launch. The effects were asymmetrical as
a function of scan position, but could be corrected by applying a simple
2x2 matrix operation to convert "measured" vertical and horizontal data
to "corrected" vertical and horizontal data at each scan position. The
correction coefficients were estimated by fitting brightness
temperatures, averaged by scan angle over ocean regions between
latitudes 30 and 50 degrees South for December 1978, to modeled
functions of the azimuthal scan angle. These coefficients were assumed
to be constant in space and time, and were applied globally and for the
entire mission.

Each scan of data in the SMMR output includes a 16-bit "scan status word". Six bits of this word are set to indicate possible losses of
data quality in the data output for that scan. Details of the
interpretations of these data quality flags are described in Section 8.

Table 3: Interpretation of Scan Status Word
-------------------------------------------------------------------------------
(16-bit word --- all bits will be zero except as follows:)
Bit 0 = 1: Possible loss of data quality in level 1A processing
(flag passed from input TAT data).
Bit 1 = 1: Scan is in period of initialization of calibration and
engineering data running averages. Brightness temperature calibrations may be
less reliable during this period.
Bit 2 = 1: One or more of the instrument component temperatures used
in the radiometric calibrations has deviated by more than +/- 3K from its
running average value in the previous scan. The accuracy of the brightness
temperature calibrations in this scan may be reduced.
Bit 3 = 1: Pitch, roll, or yaw error value is greater than +/- 0.95
degrees. This indicates an anomalous spacecraft attitude variation.
Bit 4 = 1: The average value of the footprint incidence angles in the
scan has deviated by more than +/- 0.1 degrees from the average value for in
previous scan. This indicates where caution should be exercised in using
incidence angles in subsequent geophysical processing.
Bit 5 = 1: Scan is in the "sun-in-the-cold-horn" period. Cold
calibration counts have been interpolated and brightness temperature
calibration accuracy may be reduced.
(Bits 6-15 not used)
--------------------------------------------------------------------------------

Processing Steps

The steps performed in reprocessing the SMMR data from uncalibrated
sensor data (level 1A) to calibrated brightness temperatures (level 1B)
included the following:

(1) Calibration of the radiometric data to convert from raw counts
to antenna temperatures.
(2) Adjustment for calibration bias errors, long-term calibration
drifts, and calibration drifts with respect to the sun-
spacecraft (ecliptic) angle.
(3) Interpolation of the radiometric data for all channels to the
locations of the 37 GHz vertical radiometric data.
(4) Correction of the antenna temperatures for polarization mixing.
(5) Setting of flags to identify anomalous conditions and possible
losses of data quality.

The processed data were stored as orbit files in HDF format. The files
were created on a Sun SPARCstation IPX with version 3.3 of the HDF
library. They were transferred to 8-mm tape via the UNIX tar utility.
All files were compressed with the UNIX compress command.

Software Description

Software for the processing algorithms was written in FORTRAN 77 and C.
The code was compiled and run in a UNIX environment on a SUN IPX
Sparcstation. The output files are in Hierarchical Data Format (HDF).

Calculations

Special Corrections/Adjustments

Calculated Variables

Brightness Temperatures

Sources of Error

The purpose of the SMMR instrument calibration was to relate raw sensor
output (counts) to radiance received at the antenna aperture (brightness
temperature). It was desired that the calibration be accurate and
stable over the duration of the mission. To accomplish this, time-
varying effects including thermal gradients in the components,
polarization mixing in the antenna and switches, and component losses
and degradation had to be accounted for. After processing the data to
account for these effects residual errors may remain in the data, but
these are expected to be significantly smaller than the original errors.
A discussion of expected residual errors, based on analysis of the
processed data, is given by Njoku et al. (1995).

No comparisons with independent data sources have been performed.

Quality Assessment

Quality indicator flags are provided in the scan status word for each
scan. These flags indicate conditions in the original data or in the
data processing which may give rise to reduced-quality calibrated
output.

Limitations of the Data

Antenna effects:

Corrections are made for antenna sidelobes viewing space and for cross
polarization mixing. However, no corrections are made for the antenna
pattern beam shape or sidelobes within the Earth field of view. (Such
corrections are difficult to perform for the SMMR due to the variable
antenna pattern and sampling characteristics across the swath (Njoku 1980).
Attempted improvements may be of questionable accuracy when applied
globally. Hence, it has been left at the discretion of the user to
perform this step in subsequent processing if the improvement obtainable
appears to justify the effort involved.) Thus, although the term
"brightness temperature" is applied to this level 1B product, it should
be kept in mind that these data are smoothed versions of the actual
brightness temperatures. The smoothing functions are the antenna
patterns, whose half-power beamwidths provide the equivalent footprint
dimensions at each frequency as shown in Table 1.

Operational Anomalies and Missing Data:

Acquisition of Nimbus-7 SMMR data commenced on October 25, 1978. The
SMMR operated continuously during a three-week checkout period from
start-up until November 16, 1978, at which time it began alternate-day
operation. Switching of the instrument on and off on alternate days was
the normal mode of operation for the SMMR for most of its mission due to
power sharing constraints among instruments on the spacecraft. A
special operations period occurred from April 3 to June 24, 1986, during
which the SMMR was switched on and off more frequently, with "off"
periods averaging 75 minutes and "on" periods averaging 30 minutes. The
antenna scanning mechanism was turned off on August 20, 1987 marking the
end of the SMMR data set.

Time gaps in the SMMR data of varying durations occurred during the
mission. The table below summarizes the total instrument "on" and "off"
times each year, with percentage estimates of missing data during "on"
time. The tabulated values were estimated from data times recorded on
the input TAT tapes. The high percentage of data missing during 1987 is
due to the presence of several large data gaps in the 8- to 20-hour
range. At this stage in the mission, the Nimbus-7 spacecraft had begun
to exhibit power supply degradation, and the instrument on-off cycling
modes were changed to conserve power and to focus on priority science
objectives between instruments.

Any missing data within an orbit file are indicated by zeros in all data
fields for the corresponding scan.

The data on the input TAT tapes contained some periods in which all
antenna angle values were zero. This condition impeded proper
calculation of the output brightness temperatures since the data
interpolation step in the processing require accurate antenna angles.

All occurrences of zero-value antenna angles were in 1980 on the days
shown below. No data were processed on these dates:

January 5,7,9

February 28

March 1,3,17,19,21

April 10,12

Known Problems with the Data

Although corrections applied to the data compensate for most of the
long-term drift and in-orbit variations, some residual errors may still
exist. Additional fine-tuning iterations and small-scale adjustments
may be necessary for applications requiring higher precision in
calibration than could be provided here, for instance, for sea surface
temperature measurement (Njoku et al. 1995).

Errors in the input engineering data, which affect output brightness
temperatures through the calibration equation, are simply flagged by the
processing software. These flags can be used to screen bad data
if necessary in further processing. Major deviations of brightness
temperature from expected values can usually be traced to anomalous
instrument component temperatures. For example, high 37 GHz horizontal
brightness temperatures for June 19, 1979 can be attributed directly to
unreliable calibration horn waveguide temperatures.

Individual instances of anomalously high or low brightness temperatures
in a scan are usually traceable directly to "bad" or "questionable"
antenna counts in the input TAT data. No provision has been made to
screen out these scattered anomalies since it is best left at the user's
discretion to place error-bar filters on the brightness temperature
data. There is a high frequency of this type of error during 1986,
especially during and for some time after the Special Operations Period
(April - October). Interpolating radiometric samples within scans and
between adjacent scans tends to smear the effect of these "bad" antenna
counts, which are most noticeable in browse image maps of 6.6 GHz
horizontal polarization data.

Usage Guidance

It is recommended that users should apply the offsets given in Table 2, and should use the quality indicator flags given in Table 3 to examine
the data for possible screening. Users should also observe that the
footprint incidence angles, though nominally constant at 50.3 degrees,
do exhibit small variations with scan position, position in orbit, and
with time during the mission (especially the jump in January 1984). The
characteristics of these variations have been described by McMillan and Han (date TK), and should be taken into account for estimations of geophysical
parameters that are sensitive to the brightness temperature dependence
on incidence angle.

Application of the Data Set

Key geophysical variables that can be derived from the SMMR Brightness
Temperature (Level 1B) Dataset include: